RNA interference or “RNAi” is a process of sequence-specific down-regulation of gene expression (also referred to as “gene silencing” or “RNA-mediated gene silencing”) initiated by double-stranded RNA (dsRNA) that is complementary in sequence to a region of the target gene to be down-regulated (Fire, A. Trends Genet. Vol. 15, 358-363, 1999; Sharp, P. A. Genes Dev. Vol. 15, 485-490, 2001).
Over the last few years, down-regulation of target genes in multicellular organisms by means of RNA interference (RNAi) has become a well established technique. In general, RNAi comprises contacting the organism with a double-stranded RNA fragment (generally either as two annealed complementary single-strands of RNA or as a hairpin construct) having a sequence that corresponds to at least part of a gene to be down-regulated (the “target gene”). Reference may be made to International application WO 99/32619 (Carnegie Institute of Washington), International application WO 99/53050 (Benitec), and to Fire et al., Nature, Vol. 391, pp. 806-811, February 1998 for general description of the RNAi technique.
In nematodes, RNAi can be performed by feeding the nematode with the RNAi fragment or with a bacterial strain that either contains the RNAi fragment or that upon ingestion by the nematode is capable of expressing the RNAi fragment. For a description of this so-called “RNAi by feeding”, reference may be made to International application WO 00/01846 by the present applicant, to 1998 East Coast Worm Meeting abstract 180—Timmons and Fire “Creation Of Hypomorphic Pseudo-Mutants Via Bacterial-Mediated RNAi.” East Coast Worm Meeting (1998), and again to WO 99/32619.
RNAi has also been proposed as a means of protecting plants against plant parasitic nematodes, i.e. by expressing in the plant (e.g. in the entire plant, or in a part, tissue or cell of a plant) one or more nucleotide sequences that form a dsRNA fragment that corresponds to a target gene in the plant parasitic nematode that is essential for its growth, reproduction and/or survival. Reference may be made to U.S. Pat. No. 6,506,559 (based on WO 99/32619), column 11, line 55 to column 12, line 9 and column 13, line 61 to column 14, line 11), International application WO 00/01846 by the present applicant, page 7, lines 11-8, and International applications WO 01/96584, WO 01/37654 and WO 03/052110 for a description of such techniques.
Elbashir et al. (Nature, 411, 494-498, 2001) have demonstrated effective RNAi-mediated gene silencing in mammalian cells using dsRNA fragments of 21 nucleotides in length (also termed small interfering RNAs or siRNAs). These short siRNAs demonstrate effective and specific gene silencing, whilst avoiding the interferon-mediated non-specific reduction in gene expression which has been observed with the use of dsRNAs greater than 30 bp in length in mammalian cells (Stark G. R. et al., Ann Rev Biochem. 1998, 67: 227-264; Manche, L et al., Mol Cell Biol., 1992, 12: 5238-5248). Thus, RNAi has been proposed as an alternative to the use of antisense technology for specific down-regulation or gene silencing of target genes in mammalian cells.
Although the technique of RNAi has been generally known in the art in plants, nematodes and mammalian cells for some years, to date little is known about the use of RNAi to down-regulate gene expression in fungi.
Kadotani et al. (2003) Mol Plant Microbe Interac. 16: 769-776 describe RNA-mediated gene silencing in the ascomycete fungus Magnaporthe oryzae (formerly Magnaporthe grisea; anamorph Pyricularia oryzae Cav. and Pyricularia grisae), the causal agent of rice blast disease, by a mechanism having molecular features consistent with RNAi. Gene silencing was achieved by expression of dsRNA inside cells of the fungus: fungal protoplasts were transformed in the laboratory using DNA constructs capable of expressing the double-stranded RNA, such that the double-stranded RNA is transcribed within cells of the fungus.
Cogoni and Macino, (1999) Nature. 399: 166-169 describe gene silencing by RNAi in the filamentous fungus Neurospora crassa. Gene silencing was achieved by transforming fungal cells with a transgene capable of expressing the double-stranded RNA, allowing the double-stranded RNA to be transcribed within cells of the fungus.
Liu et al. (2002) Genetics. 160: 463-470 describe RNA interference in the human pathogenic fungus Cryptococcus neoformans. Again, RNAi was achieved by transforming fungal cells in culture with a DNA construct encoding the double-stranded RNA, such that the double-stranded RNA was transcribed in situ in the fungal cells.
These studies confirm that RNA interference pathways are active in a number of different species of fungi. However, to date RNAi has only been achieved in fungi by transcription of dsRNA within cells of the fungus, following transformation of fungal cells with a DNA construct or transgene from which the appropriate dsRNA may be transcribed.
RNAi techniques requiring transformation of fungal cells with a DNA construct that directs production of dsRNA within the fungal cells are useful for experimental studies within the laboratory but are clearly not suitable for many potential practical applications of RNAi, for example applications which require dsRNA to be introduced into many fungal cells on a large scale or in the field, for example, to protect plants against plant pathogenic fungi or large scale treatment of substrates to protect against fungal infestation, or for pharmaceutical or veterinary use in the treatment or prevention of fungal infestation in humans or animals.